A suspension-system of this kind is known from D-D-PS No. 28 186, in which a suspension-spring and a correcting spring arranged in parallel with each other are provided. The suspension-spring possesses positive rigidity, i.e. as the spring is compressed, the force released thereby increases. This increase may be linear or approximately linear, so that the characteristic curve of the suspension-spring in the force-travel diagram is a line ascending more or less linearly towards the right. The suspension-spring is relatively stiff and permits only limited spring-travel. The suspension-spring is usually in the form of a helical spring. The correcting spring of the suspension-system is a magnetic spring and has two parts, one of which is connected to the sprung mass and the other to the unsprung mass of the vehicle, namely the wheels and axles. One part consists of an annular permanent magnet in which the north and south poles provided are separated from each other by gaps in the ring. The other parts consist of a soft-iron armature equipped with projections and indentations. The soft-iron armature itself has no magnetic properties but is magnetized by the magnetic flux of the other part, depending upon its position. When the projections from the soft-iron armature are exactly at the gaps between the north and south poles, as illustrated, this magnetic spring is in unstable equilibrium at its operating point. The projections of the soft-iron armature are attracted by adjacent poles of the permanent magnet with equal but opposite forces. A small shift or rotation of the soft-iron armature results in the unstable equilibrium position being abandoned and an increasing force, acting in the direction of movement, being applied to the projections. The same occurs in the opposite direction of rotation and again the force acting in the direction of movement increases. This force is at its maximum at an angle of rotation of about 22.5.degree., but declines to zero again when the projections on the soft-iron armature are exactly opposite the poles of the permanent magnet, i.e. after a rotation of about 45.degree.. A stable equilibrium position is then reached, i.e. rotation from this position allows a force to arise on the armature in a direction opposite to the direction of movement. Thus, the desired non-linear characteristic curve of the suspension-system is produced from the suspension-spring and the parallel correcting spring in that the pole-like projections on the soft-iron armature are attracted by the poles, each at an angle of 45.degree. thereto, of a permanent magnet. The forces produced by the attraction between the soft-iron projections and the permanent magnet are limited and are inadequate for many applications. In fact, the desired soft characteristic curve at the operating point of the suspension-system can be obtained only if the negative rigidity at the zero-crossover of the correcting spring is almost of the same magnitude as the positive rigidity of the suspension-spring. The negative rigidity of the correcting spring signifies that, with increasing deflection, its characteristic curve in the force/travel diagram declines, and, at the operating point of the suspension-system, a zero-crossover of the characteristic curve of the correcting spring, within the meaning of an unstable equilibrium, must never be provided. The zero-crossover signifies that the correcting spring releases no force at this operating point. In the case of the magnetic correcting spring in D-D-PS No. 28 186, the achievable rigidity, i.e. the magnitude of the descending slope of the characteristic curve of the correcting spring in the force/travel diagram, is very limited. Another disadvantage is that movement of the springs produces in the soft-iron armature eddy-currents leading to considerable heating and damping which must frequently be regarded as an unwanted side-effect. As a result of the reverse-magnetizing of the soft-iron armature there occurs, during spring movements, a force/travel characteristic which, with high oscillating frequencies such as are typical in motor-vehicle suspensions, has an increasing chronological phase-delay as compared with the stationary case. This may even lead to dynamic instability, which is highly detrimental. The known suspension-system must comprise a device for adjusting the zero-crossover of the correcting spring, since the permanent magnetic poles of the outer part of the ring must be adjustable in relation to the pole-like projections on the soft-iron armature, or vice-versa. More particularly when the motor-vehicle is at rest, the two parts must be adjustable to the gaps, i.e. in unstable equilibrium, depending upon the load on the vehicle.
German OS No. 1 755 496 also discloses a suspension-system consisting of a suspension-spring and a correcting spring in parallel therewith. In this case, the correcting spring is a helical spring which exhibits its maximum compression in unstable equilibrium. In this way a dead-centre position is reached. Superimposition of the characteristic curves of the suspension-spring and the correcting spring produces, for the suspension-system, the desired softness of characteristic at the operating point. Various types of adjusting devices are shown for the purpose of adjusting the dead-centre position of the correcting spring according to the load carried by the vehicle.
It is the purpose of the invention to provide a suspension-system for motor-vehicles which will be light in weight but will provide improved driving comfort as compared with existing suspensions. In addition to this, there is to be a reduction in spring-travel produced by static forces, e.g. changes in the load carried by the vehicle, travelling around curves, accelerating and braking, changes in aerodynamic loading, and the like.